A number of transmission schemes are utilized to maximize the amount of information that can be transmitted in a given bandwidth. Two popular modulation schemes are known as quadrature phase shift key (QPSK) and quadrature amplitude modulation (QAM) (in which the phase and amplitude of a modulated sine wave carrier signal are utilized to convey information). An example of a QPSK transmission scheme is illustrated in FIG. 1. QPSK signals are generated by shifting the phase of a carrier wave by .pi./2 radians. A QPSK signal has one of four possible phases, each phase representing one of four binary pairs (00, 01, 10, 11). The QPSK wave is defined by EQU S.sub.i(t) =cos(.omega..sub.ct +.theta..sub.i).
Transmission of this type is often called quadrature transmission, with two carriers in phase quadrature to one another (cosine .omega..sub.ct and sine .omega..sub.ct) transmitted simultaneously over the same channel.
Referring to FIG. 1, the horizontal axis, corresponding to a.sub.i, is called the "in phase" axis. The vertical axis, corresponding to b.sub.i, is called the "quadrature" axis. The signal points in the four quadrants of FIG. 2 represent a signal "constellation."
By assigning multiple values to a.sub.i and b.sub.i, the multi-level symbol signalling scheme known as quadrature amplitude modulation (QAM), is generated. The QAM scheme involves multi-level amplitude modulation applied independently to each of the quadrature carriers. Thus, a 16 state constellation, such as that illustrated in FIG. 2, may be generated. Each point of the QPSK modulation scheme of FIG. 1 now represents four points in the QAM scheme, so that a total of 16 points are defined in the QAM constellation. The general QAM signal is given by: EQU S.sub.i (t)=r.sub.i cosine(.omega..sub.ct +.theta..sub.i)
The amplitude r.sub.i is given by the appropriate combinations of (a.sub.i, b.sub.i). A phase detector/amplitude level detector combination is then used to extract digital information.
In both QPSK and QAM schemes, it is necessary to extract phase information so that the demodulation of the signal may be achieved. In other words, both the frequency and the phase of the incoming signal must be matched by a demodulator to accurately decode the information signal.
Errors can be introduced to a transmitted complex signal that can affect the amplitude and phase of the signal, leading to transmission errors. Amplitude errors can be minimized by use of an automatic gain control (AGC) circuit. A phase error, also known as "jitter" is a rotation of a transmitted symbol that does not change the magnitude of the symbol. This can cause a receive signal to differ from a transmitted signal, resulting in recovery errors.
FIG. 3 illustrates a jitter error that may be introduced to a complex input signal. In FIG. 3, the horizontal axis represents changes in the real portion of the complex term and the vertical axis represents changes in the imaginary portion of the complex signal. In FIG. 3, constellation points A and D are illustrated in first and second quadrants, respectively. These constellation points A and D represent the terminal of a vector whose origin is coincident with the origin of the coordinate system. A phase shift error results in the vector shifting by an angle .theta. in the direction of the phase shift so that the actual constellation points are moved from points A and D to points A' and D', respectively. The amplitude at both A and A' is equal. However, the angle with respect to the horizontal axis has changed by an amount .theta.. This phase offset error, or jitter error, is often corrected in the prior art through use of a phase locked loop.
One prior art attempt to eliminate jitter from phase encoded signals is described in U.S. Pat. No. 4,953,186 issued Aug. 28, 1990 and assigned to Silicon Systems Inc., the assignee of the present application. This prior art scheme uses a decision-directed error signal as an input to a feedback loop. The error signal is filtered and coupled to a phase locked loop centered at the center of the jitter tracking frequency range, e.g., 55 Hz. Because the jitter tracker is restricted to a constrained frequency range, tracking capability is limited.
A prior art attempt to provide dynamic capture range adjustment for a jitter tracker is described in U.S. Pat. No. 4,689,804. In the '804 patent, the capture range of a phase locked loop is dynamically altered during a training sequence to allow for capturing a wide range of jitter frequencies. During the training sequence, the damping factor of the loop is gradually altered to substantially reduce the capture range and response time of the loop once jitter acquisition has occurred. This results in enhanced noise performance.
When a training sequence is initiated, a capture range of approximately 300 Hz is defined. During the course of the training sequence, the capture range is gradually reduced to approximately 20 Hz, so that the noise bandwidth is very narrow. If the jitter increases beyond the 20 Hz range, a large number of errors will occur and a training sequence will again be initiated to widen the capture range.